Abstract
AbstractTime-resolved photometry and spectroscopy of cataclysmic variables (CV) are powerful tools for clarifying the physical size, placement, and conditions of emission regions in these compact systems. We present HST time resolved UV spectroscopic data for one self-eclipsing mCV system, ST LMi, and two secondary eclipsing systems, DP Leo and UZ For. These observations clearly show the relative contributions of three emission components: the white dwarf, a hotter region surrounding the accretion shock, and the photoionized accretion stream. UZ For was observed in an unusually active state by both HST and EUVE. These data provide strong evidence that the accretion stream is “clumpy”, with overdensities 100–1000 times the average density. Using photoionization codes such as XSTAR and CLOUDY, we find that steady, supersonic, and homogenous flow in the UZ For accretion stream is unstable to compression heating. As a consequence, we propose a two-phase model for the stream and obtain density enhancements comparable to those deduced from the strengths of the UV emission lines and EUVE absorption features. Similar behavior may be expected from active, asynchronous magnetic CVs with coupling radii greater than several white dwarf radii.
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